It is well known that the properties of pigments and fillers can be improved by depositing organosilicon compounds on their surfaces. By means of such a coating, the incorporation and dispersion of the pigments or fillers in lacquers, dispersion paints, plastics, toners, building materials, enamels or the like is facilitated. Moreover, the agglomeration of pigments and fillers during storage and processing is to be avoided by means of such a coating with organosilicon compounds. It is furthermore desirable to reduce the formation of dust during production and storage of pigments and fillers.
This state of the art is summarized in EP-A-0 373 426. The object of EP-A-0 373 426 are paint and magnet pigments containing at least 0.1% by weight and not more than 5% by weight of one or several polyorganosiloxanes, which have a viscosity of 100 to 100,000 mPa.times.sec and a relative molecular weight of 500 to 500,000, contain no reactive or cross-linking groups and contain at least one Si-alkyl and/or Si-aryl group with 9 to 25 carbon atoms per molecule, these groups constituting 7 to 70% by weight of the polyorganosiloxane, the remaining organic groups in the polyorganosiloxane having 1 to 8 carbon atoms.
The organosilicon compounds, named in the aforementioned EP-A-0 373 426, admittedly are suitable in many cases for coating paint pigments. However, they frequently fail with fillers, such as magnesium or aluminum oxides or their oxide hydrates. Frequently, a better dispersability of the coated particles and a better adhesion of the coating to the surface of the particles is also desired.
In the British patent application GB 2,244,489, a filler is claimed, which is provided with a layer of cross-linked silicone elastomers, which are bound chemically to the surface of the inorganic core. It is evident from the Examples cited there that, initially, highly cross-linked silicone elastomers with a plurality of reactive Si--OH groups (also referred to as "rubbery reactive silicones") are prepared which, after their application on the inorganic filler, must be cured on this filler for 4 hours at 250.degree. C. The silicone elastomer is the product of a cross-linking reaction between a "multifunctionally terminated" polysiloxane and a silane cross-linking agent. These coating materials are cross-linked elastomers, which are intended to improve the mechanical properties, such as the extensibility and toughness, of filled polymers.
In the EP-A-0 321 322, magnetizable particles are disclosed, which are in a matrix of a cross-linked polyorganosiloxane and can have a particle diameter ranging from 0.05 to 3 .mu.m. The polysiloxane matrix is produced by hydrosilylating vinylsiloxanes with hydrogensiloxanes in the presence of magnetizable particles. The vinylsiloxane or hydrogensiloxane may optionally contain further reactive or also non-reactive groups and, optionally, also ionogenic groups, which are linked to a silicon atom over an Si--C bond. Good dispersability in aqueous systems is said to be achieved with the magnetizable composite particles described there. The coated, magnetizable particles find use in medicine, biology and diagnostics.
In the German patents DE-B-41 40 793 and DE-B-41 40 794, polyorganosiloxanes are described, which have epoxy groups or ester groups, which are linked over carbon atoms to silicon, in combination with long-chain alkyl groups.
These are universally suitable for coating finely divided particles, such as pigments and fillers, in order to improve their dispersability in and compatibility with organic media, such as lacquers and synthetic resins.
However, at the high processing temperatures, customarily employed, for example, for incorporating pigments and fillers in thermoplastic materials or for stoving lacquers, these coating materials exhibit inadequate heat stability. This can lead to yellowing, particularly in cases where pigments and fillers are contained in high proportion.
The present invention is concerned with the technical problem of finding coating materials for finely divided particles which, on the one hand, bring about a good dispersion of the coated particles in the organic media, without causing yellowing of the end product at higher processing or treatment temperature.
Surprisingly, it was found that these side effects can be prevented essentially by replacing the long-chain alkyl groups of DE-B-41 40 793 and DE-B-41 40 794 by alkaryl groups.